Primary Factors behind the Heavy Rain Event of July 2018 and the Subsequent Heat Wave in Japan

Abstract

An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and largescale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.

Environmental Factors for the Development of Heavy Rainfall in the Eastern Part of Japan during Typhoon Hagibis (2019)

“…At the Hakone station, the daily rainfall on 12 October was 922.5 mm, which broke the daily rainfall record in Japan. In contrast to the July 2018 heavy rainfall in central and western Japan, which lasted more than 72 hours (Shimpo et al 2019), the heavy rainfall by Typhoon Hagibis was mostly concentrated within 24 hours. In other words, this typhoon-induced heavy rainfall is characterized as concentrated rainfall with stronger intensities but shorter duration than found in the July 2018 heavy rainfall.…”

Diagnosing Environmental Properties of the July 2018 Heavy Rainfall Event in Japan

“…According to Sekizawa et al (2019), such an extremely moist condition was generated by enhanced moisture flux from the south toward the Japanese islands, induced by anomalous oceanic evaporation due to intensified low-level winds from the surface to the 925-hPa level and positive anomaly of sea surface temperature, and the enhanced moisture convergence in western Japan. Shimpo et al (2019) indicated that low-and middle-level southwesterlies from enhanced convection in the southern East China Sea and low-level southerlies from NPSH jointly bring the extreme amount of moisture to Japan. These larger-scale circulation features generate the moist environment favorable for the heavy-rain-producing convective systems.…”

“…Liu et al (2019) examined the heat wave in northeast Asia in July 2018 and showed that northward shift of the western North Pacific Subtropical High (NPSH), stimulated by an eastward-propagating wave train originated from Europe, was the cause for the heat wave. The large-scale atmospheric conditions for the extreme rainfall and heat wave in July 2018 in Japan were investigated by Shimpo et al (2019), who documented that the extreme rainfall was caused by persistent moist airflows and Baiu frontal ascent while the heat wave was due to the enhancement of both surface NPSH and upper-level Tibetan High. In this way, large-scale envi-ronmental conditions have been described.…”

What Caused the Extraordinarily Hot 2018 Summer in Korea?

“…EA experienced an unusually hot summer in 2018 that caused severe adverse societal impacts and considerable damage to ecosystems (Imada et al 2019;Liu et al 2019;Shimpo et al 2019;Tao and Zhang 2019;Xu et al 2019). Historical temperature records were broken in many countries.…”

“…The KMA report titled 'Characteristics of heat wave current status and prospect and Changma (2018)' stated that the early withdrawal of the rainy season (Chang-ma) caused HWs on the website http://www.kma.go.kr/notify/press/reqional_list.jsp?bid=press2&mode=view&num=9701. Shimpo et al (2019) stated that 2018 rain event terminated early with the northward shift of the stationary Baiu front due to re-enhancement of the North Pacific subtropical high. Liu et al (2019a) mentioned that the northward shift of the North Pacific high is record-breaking in 2018 since 1958.…”